If you know the cross-sectional area of the pipe and you know the exit velocity of the fluid, it’s trivial to figure out the flow. This was easily done from the camera information before the cap went on. Now, you’d have to have some idea as to how thick the ring of oil gushing around the cap is, but you could probably come pretty close with the same calculation. Estimating the thickness of the oil annulus might be done from the geometry of the cap, or possibly from a view of the oil flow from a different perspective.

>>> There is no good way to measure the flow rate unless they can figure out a way to run it through a pipe. <<<

and fortunately the flow just so happens to be running through a pipe. A nice long pipe that runs through the ground.

So all they have to do is to get the sensors down into the existing pipe, before the leak, and voila, they have the answer between how much is getting into the boat and how much is going into the ocean.